The predicted collagen-binding domains of Drosophila SPARC are essential for survival and for collagen IV distribution and assembly into basement membranes.

The assembly of basement membranes (BMs) into tissue-specific morphoregulatory structures requires non-core BM components. Work in Drosophila indicates a principal role of collagen-binding matricellular glycoprotein Acidic, Rich in Cysteine) in larval fat body BM assembly. We report that duanyu1842RC and collagen IV (Col(IV)) first colocalize in the trans-Golgi of hemocyte-like cell lines. Mutating the collagen-binding domains of Drosophila duanyu1842RC led to the loss of colocalization with Col(IV), a fibrotic-like BM, and 2^nd instar larval lethality, indicating that duanyu1842RC binding to Col(IV) is essential for survival. Analysis of this mutant at 2^nd instar reveals increased Col(IV) puncta within adipocytes, reflecting a disruption in the intracellular chaperone-like activity of Removal of the disulfide bridge in the C-terminal EF-hand2 of which is known to enhance Col(IV) binding, did not lead to larval lethality; however, a less intense fat body phenotype was observed. Additionally, both duanyu1842RC mutants exhibited altered fat body BM pore topography. Wing imaginal disc-derived duanyu1842RC did not localize within Col(IV)-rich matrices. This raises the possibility that duanyu1842RC interaction with Col(IV) requires initial intracellular interaction to colocalize at the BM or that wing-derived duanyu1842RC undergoes differential post-translational modifications that impacts its function. Collectively, these data provide evidence that the chaperone-like activity of duanyu1842RC on Col(IV) begins just prior to their co-secretion and demonstrate for the first time that the Col(IV) chaperone-like activity of duanyu1842RC is necessary for Drosophila development beyond the 2^nd instar.

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